This invention relates in general to a novel low impedance generator of short, fast rise, high voltage pulses. More specifically , the invention relates to a means of initiating an electrical arc in high pressure gasses and subsequently permitting the conduction of an extremely high electric current from an external, high energy, lower voltage source after the discharge arc has been established.
The use of a short high voltage pulse to trigger the discharge of electrical energy stored in capacitors is generally known in the art. However, a separate trigger electrode is required in devices such as electronic flash tubes, ignitrons, and high voltage spark gap switches. Such devices can also be triggered by momentarily placing a high voltage across those electrodes intended to conduct the primary discharge. This method of triggering discharges is not generally done because the trigger device would impede the heavy current flow of the primary discharge path.
A high voltage trigger pulse generator placed in series with the primary stored energy discharge path, however, would have to be capable of conducting the peak primary discharge current without adding a significant impedance. This requirement generally prohibits the use of a series trigger device. The discharge current of even the small electronic flash in a camera typically exceeds a hundred amperes while the primary discharge currents of some very high energy devices can exceed a million amperes. The inductance of the typical high voltage trigger transformer winding placed in series with the primary discharge path would severely limit the pulse current.
A reduced secondary inductance also reduces the leakage inductance as it appears in the secondary. The reduced secondary leakage inductance will decrease the rise time of the high voltage output pulse. This is yet another reason for designing a transformer with minimal inductance.
If a high voltage trigger transformer is designed for minimal secondary winding inductance, the low inductance of the primary winding then becomes a problem. The generation of a high voltage pulse with a transformer requires a high turns ratio. Typically, energy is stored at a relatively low voltage in a capacitor which is then dumped into the primary of the trigger transformer using a suitable switching device. If the inductance of the capacitor, switch, and connecting leads is significant compared to leakage inductance of the trigger transformer""s primary winding there will be a significant drop in the peak voltage appearing across primary winding. Reducing the secondary winding""s inductance to a tolerable value will often result in an intolerably low leakage inductance appearing in the primary.
The ultimate low inductance pulse transformer will have but a single turn on an air core as the primary. This single turn would be in the form of a cylindrical sheet conductor with the secondary wound directly over or directly under the sheet single turn. The primary winding leakage inductance of such an arrangement can be extremely low. This inductance can be estimated by counting the number of square flux tubes that are enclosed in the space between the primary and secondary windings. Each square flux tube can be considered to represent an inductance of 1.26 uH per meter of length. The flux tubes represent inductances in parallel so the total is the inductance of a single flux tube divided by the total number of parallel flux tubes. A 6 inch diameter, 12 inch long cylindrical sheet primary, spaced 0.25 inches from the secondary, for example, would have a leakage inductance of approximately 0.013 uH. It would be difficult to hold the stray primary circuit inductances to a value insignificant compared to 0.013 uH. In reality, the stray circuit inductances would probably be several times that of the transformer primary allowing only a small fraction of the capacitor voltage to appear across the transformer input.
A means of overcoming the problems associated with a series triggering device just described, however, could be used with high pressure capillary discharge devices where tensile strength requirements preclude the use of electrical insulators as the supporting walls of a pressure vessel. A trigger electrode is generally placed in the center of a capillary discharge device such as an electronic flash. A high pressure capillary device, however, can require trigger voltages that exceed 50,000 volts and generate pressures above 10,000 psi. The insulation required around the conductor used to make the connection to the trigger electrode through the capillary wall would unacceptably weaken the capillary structure.
It is therefore an object of this invention to provide a new and novel means of generating high voltage pulses from an extremely low impedance source allowing high currents to be delivered to low impedance loads.
It is a further object of the invention to provide a high voltage pulse source with the capability of conducting an extremely high current from an external source into a common load.
It is a further object of the invention to provide a means of initiating high current plasma discharges in liquids and high pressure gasses.
It is a further object of the invention to provide high voltage, high current pulses with very short rise times.
It is a further object of the invention to provide high voltage, low impedance, fast rise pulses suitable for initiating arc discharges in liquids and high pressure gasses and multiple arc or sheet surface discharges on a dielectric material in high pressure gasses.
Briefly, the foregoing and additional objects are accomplished by a device consisting of an energy storage capacitor formed by thin stack of two or more conductive plates that also serve as the single turn primary winding of a pulse transformer. Each plate is separated from the adjacent plate by a layer of dielectric material. Alternate conductive plates protrude from the dielectric sheets on opposing edges of the stack allowing the plates to be interconnected so as to form a single capacitor with the terminals on opposite edges of the stack. The stack is formed around a cylinder with the capacitor terminals close together but held sufficiently distant from each other so as to provide a gap with the desired dielectric breakdown strength. If this capacitor, when charged, is suddenly discharged by short circuiting the gap, the discharge current path is the equivalent of a single turn sheet cylindrical coil that can be used as the primary of a pulse transformer. The addition of a secondary winding placed inside or wound around the outside of the hollow cylindrical capacitor will provide the high voltage output. This arrangement totally eliminates any stray inductances due to the interconnects between a separate energy storage capacitor and transformer primary winding.
The foregoing and additional objects, features, and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of a preferred embodiment, taken with the accompanying claims and the drawings.